Device for controlling discharge rates from tanks



May 30, 1950 c. A. DE GlERS ETAL DEVICE FOR CONTROLLING DISCHARGE RATES FROM TANKS Filed Dec. 22, 1945 IINVENT RS ATTORNEY Patented May 30, 1950 DEVICE FOR CONTROLLING DISCHARGE RATES FROM TANKS Clarence A, de Giers, Forest Hills, and Rolf A. F. Sandberg, Queens Village, N. Y., assignors to The Liquidometer Corporation, Long Island City, N. Y., a corporation of Delaware Application December 22, 1945, Serial No. 637,046

13 Claims. 1

This invention relates to control devices or systerns for controlling the rate of discharge of liquid or other fluid from a. plurality of containers, and particularly to control devices or systems for controlling the rate of discharge of fuel from a plurality of tanis on an aircraft.

For certain types of installations and devices in which liquid is drained from a plurality of containers it is of importance that the rate of discharge of liquid from the containers be uniform. The invention may be applied to aircraft such as single motored combat aircraft sub ected to considerable and swift maneuvering. Here it is essential that the fuel weight distribution be kept in balance at all times in order to obtain the best performance of the aircraft. In multimotored aircraft, it is also of importance to equalize the unbalance of the tanks which may result from unequal fuel consumption of the engines.

Accordingly, it is one of the objects of the invention to provide a novel and improved control device or system for equalizing the distribution of fuel in several containers or tanks.

Another object of the invention is to provide a novel and improved control device or system for regulating the flow of liquid from one tank corresponding to the liquid contents of one or more other tanks.

Another object of the invention is to provide a novel and improved control device or system which is self-regulating and which automatically maintains substantially uniform contents and hence a weight balance in the various tanks to be supervised.

Another object of the invention is to provide a novel and improved control device or system for the purposes aforesaid, which remains inactive 1 when and while the contents or weight distribution in the tanks to be supervised are in balance and becomes active only when and while such balance is disturbed, such arrangement having the advantage that the control device or system is at rest as long as the weight and contents distribution are in balance, one result being that wear and tear of the movable parts of the control device or system are reduced to a minimum.

Other and further objects, features and advantages of the invention will appear hereinafter and in the appended claims forming part of the application.

In the accompanying drawing the single figure shows a now preferred embodiment of an electrical control device according to the invention, including a circuit diagram, said drawing being an illustration of the specification and not to be 2 construed as imposing limitations on the invention.

A control device as shown in the figure may be successively used wherever it is necessary to control the discharge of liquid from two or more tanks. It is particularly useful to secure the uniform drainage of fuel tanks of combat aircraft. As previously pointed out, it is essential on such aircraft that the fuel weight distribution be kept in balance at all times in order to obtain the best performance from the aircraft when maneuvering.

There are diagrammatically shown in the figure two fuel tanks 5 and 2 which may be the wing tanks of an aircraft and which feed the plane engine (not shown) through a main pipe 3 to which they are connected by feeder pipes i and 5 respectively. The rate of flow of fuel from the tanks through pipes t and ii is controlled by conventional valves 6 and i respectively. The opening of each valve is controlled by the electromagnets 8, 3 respectively. The energizing circuits of these electromagnets are in turn controlled by a control system according to the invention which will be now described in detail.

With each tank, the drainage of which is to be supervised, there are associated two resistance elements it, if for tank 5 and l2, it for tank 2 respectively. Resistance element ill is engaged by a slider contact i i and resistance element H by a slider contact 55. The position of both slider contacts relative to the respective resistance elements is controlled by the liquid level it in tank I. For the purpose aforesaid, sliders i l and i5 are mounted on a common insulating support 5'! pivotally supported by a pivot iii. Support !'l is operatively connected by a mechanical transmission means it to a float arm 26 fastened to a float 25 floating on the liquid, the level it of which is to be supervised. Float arm 2%] is pivot ally supported at pivot 22 by the walls of tank 6. The opening in the tank through which fioat arm 29 is introduced into the tank may be closed by means of a seal of the bellows type (not shown) permitting, a rocking movement of float arm 2%? and hence a corresponding angular movement of support il in response to a change of the liquid level in the tank. Float and linkage systems as previously described are Well known in the art and therefore not illustrated in detail. However, it should be noted in this connection that any suitable, conventional float and linkage system may be employed to control the positions of sliders it and i5 relative to the respective resistance elements.

The resistance elements it and i3 are slidably engaged by slider contacts 25 and 26 respectively supported on a common support 21. This support is pivotal about a pivot point 28 and con nected by a mechanical transmission 29 to a float arm 38 supporting a boat 3| floating on the liquid in tank 2, the level 32 of which is to be supervised. Float arm 30 is pivotally supported at 33 by the walls of tank 2. The opening in the tank walls for float arm 30 may be closed by a seal of the bellows type (not shown).

The control system according to the invention is energized by means of a source of current such as a battery 40. The negative terminal of this battery is grounded by a lead 4| and the positive terminal is connected by a lead 42 and a control switch 43, which is closed during operation of the control system, to a supply lead 44. This supply lead is connected by leads 45 and 46 to one end of resistance elements In and I3 respectively. The slider |4 engaging resistance element I is connected by 'a pigtail 41 and a lead 48 to one end of resistance element [2. Slider 25'engaging this resistance element is connected by a pigtail 49 and a lead 50 to one terminal of a heater coil the other terminal of which is grounded .by a lead 52. Similarly, slider 26 engaging resistance element I3 is connected by a pigtail 58 and a lead 54 to one end of resistance element The slider |5 engaging this resistance element is connected by a pigtail 55 and a lead 56 to one end of a heater coil 51, the other end of which is grounded by a lead 58. As will now be apparent, an angular movement of support |1 about pivot l8, say for instance in counter clockwise direction in response to a fall of the liquid level in tank I, will cause an increase of the effective resistance of resistance element I0 and a decrease of the eifective resistance of resistance element Correspondingly, an angular movement of support 21 about pivot 28 in response to a fall of the liquid level in tank 2, which will impart a clockwise movement to support 21 due to the reversed arrangement thereof, will cause a decrease, of the effective resistance value of resistance element l2 and an increase of the effective resistance of resistance element l3. In other words, a change of the liquid level in the same direction in both tanks will result in an increase of the resistance value of the resistor of one tank and in a decrease of the resistance value of the associated and series-connected resistor at the other The resistance elements and the rate of transmission of the float and linkage systems at both tanks are so selected that an equal rise or fall of the liquid levels in both tanks will result in an equal increase and decrease of the resistance values of the associated resistors, or in other words, the total resistance values of the resistors remain unchanged. Consequently, the currents through heater coils 5| and 51 will be equal when the liquid levels in both tanks change uniformly. It will further be obvious that the decrease and increase of the resistances of associated resistors will not compensate each other when the liquid levels in both tanks difier from each other, that is when the tanks are not drained uniformly. In such case the currents through the heater coils are at variance and the coils are heated difierently.

The heater coils 5| and 51 coact with bellows type elements 68 and 6| respectively. Each bellows has an extension 52 and 63 respectively about which heater coils 5| and 51 are wound. The extensions 52 and 63 may be hollow to accelerate the transfer of heat from the extensions to the liquid in the beifows. It is, of course, also possible to wind the heater coils about the main chamber of the bellows or to transmit the heat generated by the heater coils to the bellows by any other suitable means. .The bellows are pivotally supported on fixed pivots 64 and 65 respectively. They are either totally filled or filled with a volatile liquid, so that a vapor space is created in extension chambers 62 and 63. When the bellows are filled with a volatile liquid, the connection between the bellows and the respective extension chambers is so arranged that a vapor space forms in the warmer extension chambers 62 and 63. Under certain circumstances the use of volatile liquid filled bellows is preferable since such bellows show a greater rate of expansion or contraction in response to a given change of temperature of the bellows. oi the bellows and hence the expansion and contraction thereof is controlled by the heat generated by heater coils 5| and 51, which in turn are controlled by the liquid level changes in tanks I and 2 as previously explained. It has been found that the contraction and expansion of the bellows in response to a given change of heat generated'is influenced to a certain degree by the altitude, one reason being the lowering of the heat radiated at higher altitudes.

In order to compensate for differences in the reaction of the bellows at different altitudes and to secure a more uniform expansion or contrac tion, shunt resistors generally designated 10 and 1| may be provided. Altitude shunt 18 comprises a resistance element 12 and a slider contact 13. The relative position of the slider 13 is controlled by the expansion and contraction of an evacuated bellows 14 stationarily supported. Bellows 14 will be more or less compressed in response to the varying air pressure at difierent altitudes and regulate the resistance element 12 parallel to heater coil 51 correspondingly, thereby compensating the influence of the altitude upon the reaction of bellows 6|.

Similarly, altitude shunt 1| comprises a resistance element 15, a slider contact 16 and a stationarily supported evacuated bellows 11. Instead of employing automatically controlled altitude shunts it is, of course, also possible to provide manually adjustable shunts such as variable resistors.

Each bellows 60 or 6| is pivotally connected to a temperature compensation link pivotally supported at a point 8| by an arm 82 which is pivoted on a fixed pivot 83. Arm 82 is extended to form a pointer 84 which cooperates with a stationary scale 85. Pointer and scale positions are so arranged and selected that the pointer will be in the center position of the scale (position as shown in the figure) when the liquid levels in both tanks are equal and will deviate toward the left or right section of the scale in response to unequal liquid levels due to one or the other of the tanks draining at a faster rate. The arm 82 is rigid with an insulation bar 86 having at its ends contact arms 81 and 88. Contact arm 81 is connected by a pigtail 89 and a lead 90 to one terminal of a relay coil 9|, the other terminal of said relay coil being connected by a lead 92, a lead 93, a resistor 94, a relay contact point of a second relay, a relay armature contact 96 engaging a back contact point 95 when the relay coil 91 controlling armature contact 98 is deenergized, and a lead 98 to ground. Similarly, contact arm 76 88 is connected by a pigtail I00, a lead ||l| to one The temperature terminal of relay coil 91, the other terminal of said relay coil being connected by a lead I02. a resistor I03, a contact point I04 of relay 9I, an armature contact I05 controlled by relay 9I and engaging back contact point I04 when relay 9I is deenergized, and a lead I01 to ground.

Contact arms 81, 88, preferably madeof flexiblc material, coact with two sets of contacts H0, III for contact arm 81 and H2, H3 for contact arm 88 respectively. Contacts H0 and III are It will be readily seen that in this position of the contact arms relay coils 9I and 91 are short-circuited, the short-circuit of relay coil 9| may be easily traced from one terminal of relay coil 9i through leads 92, H6, contact H2, contact arm 81, pigtail 89, and lead 90 to the other terminal of relay coil 9|, and the short-circuit of relay coil 91 from one terminal thereof through leads I02, II1, contact H3, contact arm 88, pigtail I00 and lead IOI to the other terminal of this relay coil. Armature contact I05 of relay 9I engages a fixed relay contact I20, when the relay coil 9I is' energized and second armature contact I2I of relay 9| engages a fixed relay contact I22 when the relay coil Si is energized.

Similarly, armature contact 96 of relay 91 engages a fixed relay contact I25 when the relay coil 91 is energized and a second armature contact I26 controlled by relay coil 91 engages a fixed contact point I21 when the relay coil 91 is energized.

Contact point I20 is connected by a lead I28 to one terminal of electromagnet coil 8 controlling valve 6, the other terminal of this coil being connected to supply lead 44 as previously mentioned. Armature contact I2I is connected by a lead I29 to supply lead 44, and contact point I22 is connected by a lead I30 to lead 90 extended to contact arm 81.

Contact point I25 of relay 91 is connected by a lead I3I to one terminal of electromagnet coil 9 controlling valve 1, the other terminal of this coil being connected to supply lead 44 as previously mentioned. Armature contact I26 is connected by a lead I32 to supply lead 44, and contact point I21 is connected by a lead I33 to lead |0I extended to contact arm 88.

One of the purposes of providing relays 9| and 91 for controlling the circuit connections of coils 8 and 9 rather than to control coils 8 and 9 directly by contact arms 81, 88 and the contacts coacting therewith is to protect these contacts and the contact arms against the comparatively high currents required for the operation of coils 8 and 9. Relays SI and 91 may therefore be described as power relays. However, it should be understood in this connection that under certain circumstances the power relays may be omitted and the valve actuating coils 8 and 9 be controlled directly by the contact arms 81 and 88,

The valve actuating coils 8 and 9 and the valves 6 and 1 controlled thereby are so designed that the valves are open permitting a drainage of tanks I and 2 through pipes 4 and 5 when coils 8 and 9 are deenergized. As a result, in case of power failure, both tanks will continue to drain. The operation of a control device or system as hereinbefore described is as follows:

Let it be assumed that control switch 43 is closed, thereby rendering the control device operative and that tanks I and 2 drain uniformly so that the relative liquid levels in the tanks remain unchanged. Then, as previously described, change of the resistances of resistance elements I0 and II in response to the liquid level change in tank I will be compensated by corresponding changes of the resistances of resistance elements I2 and I3 in response to a liquid level change in tank 2. Consequently, heater coils 5i and 51 will be equally heated and the bellows elements controlled thereby will expand or contract to the same extent. Such expansion or contraction of the bellows will cause a rotation of temperature compensating link about pivot 8|, but the posi tion of arm 82 will remain unchanged. Pointer 84 will remain in the illustrated center position, thereby indicating that both tanks drain uniformly. Relay coils SI and 91 are short-circuited b the engagement between contact arms 81, 88 respectively and contacts H2, H3 respectively as previously explained. Coils 8 and 9 are deenergized and valves 6, 1 are completely or partly opened according to the adjustment of the valves.

Let it now be assumed that the two tanks drain at a difierent rate, for instance, that tank 2 drains faster than tank I. Then, the resistance changes in response to th liquid level changes are no longer compensating and one of the heater coils will be heated more than the other. Under these circumstances the float and linkage systems displacing ihe respective sliders are so arranged that heater coil 5| is now heated more than heater coil 51. Then, bellows 80 will expand more than bellows 6I. This differential heating of the bellows will impart a rotative movement to link 80 and also displace pivot BI toward the left, thereby swinging arm 82 about its pivot 83, provided.

of course that the difference in the heating of the two bellows exceeds a certain minimum. The difference in heating of the bellows to which the control device responds can be adjusted by proper selection of the lengths of the various links and arms and of the distances between contacts H8, H2 and III, II3 respectively. The swinging movement of arm 82, will caus insulation bar 85 to disengage contact arm 88 from contact I I3 and to move it into engagement with contact iii. Contact arm 81 and contact H2 remain in engagement, the resilience of the contact arms 81, '88 allowing such movement of bar 83. Consequently, relay coil 9| remains short-circuited and an energizing circuit for rela coil 91 is established which may be easily traced from the supply lead 44 through lead I I 5, contact I I I, contact arm 88, pigtail I00. lead IOI, coil 91, lead I02, resistor I03,-contact I04, armature I05 of deenergized relay SI, and lead I01 to ground. Relay 81 now attracts its armature contacts 96 and I 28 into engagement with contact points we and I21 respectively. Consequently, a relay holding circuit is closed which may be traced from supply lead 44 through lead I32, armature I26, contact I21,

.lead I33 and lead IOI, and is then continued as previously described. The purpose of this holding circuit is to retain relay 91 energized when contact arm 88 and contact Iii are temporarily disengaged, for instance, due to vibrations after contact between arm 88 and contact I i i has been once established and relay 91 has responded.

75 The energization Of relay 91 further closes a circuit for coil 9 which may be traced from supply lead 44 through coil 9, lead it 4, contact ltd, armature contact 86, and lead 99 to ground. Consequently, coil 9 is energized and closes the valve l more or less as adjusted, thereby reducing the rate of flow at which tank 2 is drained. The displacement of pivot 8| will move pointer 8t into the section of scale 85 assigned to tank l thereby indicating that tank I is the one which drains slower than the other tank.

As soon as the liquid levels in the tanks are equalized, the effective resistances of the resistance elements will again compensate each other. Consequently, both heat r coils ti and 56 will be heated equally and hence both bellows will be expanded to the same extent. Arm t2 and with it the contact arms and pointer ti now return into the position shown in the drawings in which both relay coils 9i, 9! are short-circuited and coils 8 and 9 are deenergized.

It should be noted that in the event both contact arms 87, 88 engage the respective contacts 0, HI simultaneously due to some defect in the system, the circuit arrangement prevents any energization of both relays 95, ill thereby shutting off both tanks since the energization of one relay will always interrupt the circuit of the other at contacts NM or 95.

The operation of the control device is the same when tank 2 drains slower than tank 8, the only difference being that then coils 9i and 8 are energized in a manner which will be easily understood from the previous description.

The invention has been described in connection with an embodiment in which the rate of flow of the faster draining tank is reduced. However, it should be understood that it is quite practical to reverse this arrangement and to increase the valve opening of the slower draining tank. This can be accomplished by simply adjusting valves 6 and 1 accordingly.

A diiferential draining of tanks 5 and 2 may be caused by a variety or reasons such as unequal fuel consumption of engines fed from the tanks, by friction in the feeder pipes, etc.

While the invention has been described in detail with respect to a certain now preferred example and embodiment of the invention, it will be understood by those skilled in the artafter understanding the invention, that various changes and modifications may be made without departing from the spirit and scope of the invention and it is intended therefore, to cover all such changes and modifications in the appended claims.

What is claimed is:

1. A control system for supervising the discharge of liquid from two tanks to maintain a predetermined condition of relative liquid levels therein, comprising means responsive to liquid level in each of said tanks respectively, a mechanical differential means having movable elements respectively actuated by said liquid level responsive means and having a member movable in one direction or the other by said elements in response to difference between the liquid levels in said tanks and said predetermined condition, but maintained against movement as long as said levels are substantialliy in said predetermined condition, valve means for controlling the discharge of liquid from said tanks respectively, and electrical means controlled by the position of said member for selectively actuating said valve means in such manner as will tend to re-establish said predetermined condition oi the levels in said tanks.

2. A control system in accordance with claim 1, wherein said valve means are solenoid operated valves so arranged that said valves are open when their actuating solenoids are de-energized, and wherein said electrical means controlled by the position of said member is effective selectively to energize the solenoid associated with the valve controlling the discharge of liquid from the one of said tanks having the lower liquid level therein in respect to said predetermined condition so as at least partially to close such valve until the levels in the two tanks are substantially re-established at said predetermined condition.

3. A control system for supervising the discharge of liquid from two tanks to maintain a predetermined condition of relative liquid levels therein, comprising means responsive to liquid level in each of said tanks respectively, a first electric circuit including current afiecting means controlled respectively and oppositely by said liquid level responsive means in said tanks respectively for controlling the flow of electric current in said first circuit, a second electric circuit similar to said first circuit but oppositely connected to each of said liquid level responsive means for controlling the flow of electric current in said second circuit, both said circuits being so constructed and arranged that the currents flowing therethrough will be substantially equal throughout the range of liquid level variation from full to empty in said tanks respectively as long as the levels in said tanks are substantially at said predetermined condition, but said currents will be different from one another if the levels of liquid in said tanks differ substantially from said predetermined condition, a valve controlling the discharge of liquid from each of said tanks, and means responsive to a difference of the values of said currents, but substantially independent of simultaneous changes in both said values, for selectively actuating said valves in a manner tending to re-establish said predetermined condition of the liquid levels in said tanks.

4. A control system in accordance with claim 3, wherein said means responsive to a difference of the values of said currents comprises mechanical differential means responsive to the currents flowing in said electric circuits, a member controlled in position by said mechanical differential means and movable thereby in one direction or the other in response to an unbalance of said mechanical differential means, and electrical means responsive to the posiiton of said member for selectively actuating said valves at least to reduce the discharge from the one of said tanks having the lower liquid level therein in respect to said predetermined condition.

5. A control system in accordance with claim 3, wherein said means responsive to a difference of the values of said currents comprises fluidcontaining bellows, means responsive to the currents flowing in said electric circuits for controlling the expansion of the fluid in said bellows respectively, means mechanically interconnected to both said bellows and articulated to a movable member for controlling the position of said member in response to differential expansion of said bellows, and electrical means responsive to the position of said member for selectively controlling said valves in a manner tending at least to restrict the flow of liquid through the valve con- 7 trolling the discharge from the-one of said tanks 9 having the lower liquid level therein in respect to said predetermined condition.

6. A control system for supervising the discharge of liquid from two tanks to maintain a predetermined condition of relative liquid levels therein, comprising means responsive to liquid level in each of said tanks respectively, a first electrical circuit including rheostat resistances controlled respectively and oppositely by said liquid level responsive means in said tanks respectively for controlling the flow of electric current in said first circuit, a second electric circuit similar to said first circuit and including rheostat resistances oppositely connected to each of said liquid level responsive means respectively i or controlling the flow of electric current in said second circuit, both said circuits being so constructed and arranged that the currents flowing therethrough will be substantially equal throughout the range of liquid level variation from full to empty in said tanks respectively as long as the liquid levels in said tanks remain substantially in said predetermined condition, but said currents will be different from one another if the levels of the liquid in said tanks differ substantially from said predetermined condition, a pair of fluid-containing bellows, electric heating means in said electric circuits respectively associated with said bellows for heating the fluid therein and thereby for controlling the expansion of said bellows in response to the currents flowing in said circuits respectively, means mechanically interconnected to both said bellows and including a movable member articulated thereto for controlling the position of said member in response to differential expansion of said bellows, a valve controlling the discharge of liquid from each of said tanks respectively, and electrical means responsive to the position of said member for selectively controlling said valves in a manner tending to re-establish said predetermined condition of the liquid levels in said tanks.

7. In an electrical control device for controlling the discharge of liquids from two tanks to maintain a predetermined condition of relative liquid levels therein, in combination, a valve means for regulating the flow of liquid from each of said tanks, an electrical control means connected with said valve means for controlling each of said valve means independently, a differential relay means comprising two temperature sensitive members, a switch means controlled by the relative temperature of said members, a plurality of resistors associated with each of said tanks, each of said resistors comprising a rcsistance element and a movable contact element engaging the resistance element, a float and linkage system provided for each tank, each of said systems being controlled by the liquid level in the respective tank and operatively connected with the movable element of the resistor associated wtih the respective tank for relative movement of the respective elements, a heater resistor provided for each temperature sensitive member, a circuit means for connecting each heater resistor with one of the variable resistors of each of said tanks and a source of current, said float and linkage systems being constructed to displace the respective elements relative to each other to cause an increase of the resistance of a variable resistor associated with one tank and a decrease of the resistance of the corresponding variable resistor associated with the other tank in such a manner that the total resistance included in a heater resistor circuit remains 10 unchanged in response to uniform liquid level changes in the tanks and is changed in response to a change in the relative liquid levels in the tanks from said predetermined condition, thereby causing unequal heating of the temperature sensitive members by the heater resistors, and a second circuit means for connecting the switch means with the electrical control means and a source of current, said temperature sensitive members being constructed to move the switch means into a position in which the control means are energized when and while the temperature sensitive members have unequal temperatures, said control means being arranged to regulate the valve means toward re-establishing said predetermined condition of the liquid levels in said tanks when energized.

8. In an electrical control device for controlling the discharge of liquid from two tanks to maintain a predetermined condition of relative liquid levels therein, in combination a valve means for each of said tanks, an electrical control means provided for each valve means for controlling the operation thereof, a differential relay means comprising two temperature sensitive members, a switch means controlled by the relative temperature of said members, a plurality of variable resistance means associated with each of said tanks, a prime mover for each tank, each of said prime movers being controlled by the liquid level in one of the tanks respectively and controlling the resistance value of the resistance means associated with the respective tank, a heater resistor for each temperature sensitive member respectively, a circuit means electrically interconnecting said heater resistors with said resistance means and a source of current, and a second circuit means electrically interconnecting said switch means with each of said control means and a source of current for energizing the control means through the switch means in response to a difference in the temperature of the temperature sensitive members, said resistance means being constructed and adjusted to include an equal resistance in the circuit connections of each heater resistor in response to said predetermined condition of the relative liquid levels in said tanks, thereby causing equal temperatures of the temperature sensitive members, and to include unequal resistances in the circuit connections of the heater resistors in response to a substantial variation from said predetermined condition of the liquid levels, thereby causing different temperatures of the temperature sensitive members for energizing the control means provided for the tank in which there is a lower liquid level in respect to said predetermined condition so as at least to reduce the rate of discharge therefrom until said predetermined condition is re-established.

9. In an electrical control device for controlling the discharge of liquid from two tanks to maintain a predetermined condition of relative liquid levels therein, in combination, a valve means for regulating the flow of liquid from each of said tanks, an electrical control means provided for each valve means for controlling the operation thereof, a diiierential relaymeans comprising two temperature sensitive members, a. switch means controlled by the relative temperature of said members, a plurality of variable resistors associated with each of said tanks, each of said resistors comprising a resistance element and a movable contact elementengaging the resistance element, a float and linkage system provided for each tank, each of said systems being controlled by the liquid level in one of said tanks respectively and operatively connected with the movable element of the resistors associated with the respective tanks for relative movement of the respective elements, a heater resistor provided for each temperature sensitive member, a circuit means for connecting each heater resistor with one of the variable resistors of each of said tanks and a source of current, said float and linkage systems being constructed to displace the respective elements relative to each other to cause an increase of the resistance of a variable resistor associated with one tank and a decrease of the resistance of the corresponding variable resistor associated with the other tank in such a manner that the total resistance included in a heater resistor circuit remains unchanged in response to uniform liquid level changes in said tanks and said total resistance being changed in response to substantial variations of the liquid temperature sensitive members by the heater resistors, and a second circuit means for connecting said switch means with each of said electrical control means and a source of current, said temperature sensitive members being constructed to move the switch means into a position in which all the control means are deenergized when the resistances included in the heater resistor circuits are equal and into a position in which a predetermined one of said control means is energized when said temperature sensitive means are difierentially heated respectively, each of said control means being arranged to regulate the respective valve means toward re-establishing said predetermined condition of the liquid levels in the tanks when energized.

10. An electrical control device as described in claim 6 in combination with a plurality of adjustable resistance means each connected in shunt to one or said electric heating means.

11. An electrical control device according to claim 6, wherein said electrical means responsive 45 2,029,085

to the position of said member for selectively controlling said valves includes switch means mechanically controlled by the position of said mem-- ber for selectively controlling current flow through a pair or primary circuits for said valves respectively, relay means in said primary circuits controlling current flow through secondary circuits for said valves respectively, and electrical means in said secondary circuits for directly controlling said valves, each of said valves and the operating means therefor being constructed and arranged to allow iull discharge from the respectively associated tank when there is no electric current flowing in said secondary circuits respectively.

12. A control system in accordance with claim 1, comprising in addition, indicating means mechanically controlled by said movable elements of said mechanical differential means for indicating the presence and direction of a departure of the relative liquid levels in said tanks from said predetermined condition.

18. In an apparatus or the character described to maintain a, predetermined condition 01' relative liquid levels in two tanks, two liquid i14 supply tanks, a valve for each tank operable to regulate the flow of fuel from its tank, an electrical means for operating each valve, a float in each tank controlling the electrical means and the valve associated with the tank, and electrical means rendered eflective by a departure of the relative liquid levels in said tanks from said predetermined condition for putting said valves and their electrical operating means under the joint control of said floats to regulate the valves toward re-establishing said predetermined condition of the liquid levels in said tanks.

CLARENCE A. on GIERS. ROLF A. F. SANDBERG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,804,212 De Giers May 5, 1931 1,886,439 Wells Nov. 8, 1932 1,983,093 Montgomery Dec. 4, 1934 Sussin Jan. 28, 1936 FOREIGN PATENTS Number Country Date 349,787 Germany Mar. 8, 1922 

